Modern Vespa

Hey,

How I can tell if I need less air or more fuel on a carb ?

Melted piston is a giveaway.

Aren't those sorta the same thing?

The color of the spark plug tells you the truth. Black a lot of gasoline, gray had gasoline and a lot of air, red as brick is right. There are tables where this is shown.

Color Spark plug

I'll try to explain better.

I'm asking about tuning a lean mixture on carb setup, you can add more fuel or you can block some air - right ?

In normal stock engine you would want to add more fuel - assuming the air volume is correct for the stock engine - all stock parts.

I run a modified Gilera Runner VXR200 4T, big bore big head and all.
Malossi say you should drill some holes in the air box in order to get a good mixture - they even sell bushings to put in the hole you drill.

On the fuel side you need to install a bigger jet to get the right mixture.

NOW !

If the mixture is lean - how do I know if I have too much air or too little fuel for the setup ?
What is the max "correct" air volume for the carb ?
What is the max "correct" fuel volume for the carb ?


I tune turbo engine but this is my first carb.

Lean modified engine, more fuel or less air ? How do I know the optimum ? (dyno ?)

Thanks !

It's about tuning and rehearsals, I would change the nozzle if it is too big to find out on the driving and color of the Spark plug. I would leave the air filter the original. You must be patient. I don't know if I helped you.

You haven't increased the displacement by all that much. And the valves are larger, so the bike gets more air and fuel mixture than previously. So ride it, and after 40-50 miles, check the color of the plug. The odds are that you don't need to rejet.

I have a wideband installed

What does it say?

Lean

Isn't the overall aim to get as much mixture into the chamber as possible?

Pull your plug and compare to the chart Puno posted above. Adjust accordingly.

NO, getting the right amount and the correct ratio is.

10x, that what I'm aiming for, I can add air up to a point I remove the air box completely but that would not be ideal.

What is the ideal amount and how do I find it - with out a dyno.

to set the mixture:
1) warm the engine up by driving it for 10 min or more
2) put bike on stand and turn the idle screw down as much as possible
3) adjust mixture screw so idle is at highest point
4) turn down idle screw
5) adjust mixture screw again same to seek highest idle
repeat

Not asking about idle - I have a wideband

I agree that the correct ratio of air fuel is paramount, thats why i refered to it as mixture. I must be missing the point of needing the 'right' amount as opposed to as much as possible. surely if its a tuned engine, you're building it for speed and power rather than economy and more air/fuel gives you a bigger bang.

I can run without an air box for maximum air volume but the torque will be lower...

Try a thin red sponge from Malossa to the original Air Box, a carburetor to a larger nozzle by at least 30 % and watch the ride and Spark plug.

The rules in the old days with carb engines:

For optimal fuel efficiency:
Then you need to make sure every molecule of fuel is burned. Therefore you need a mixture with an excess of oxygen (air): a lean mixture. If I remember correctly some 20% excess of air is optimal.

For maximum power:
The amount of air the engine can aspirate is the limiting factor for power. This means you need to make sure every molecule of oxygen aspirated is burned.
So never reduce the amount of air, instead try to improve air flow and then increase the amount of fuel: a rich mixture. If my memory is correct some 20% excess of fuel is optimal.

There are four main ways to add fuel with a carb:
1. adjust air fuel screw
2. adjust the needle clip (if equipped)
3. add bigger idle jet
4. add bigger main jet

Depending on what your wideband says and when it says it. I would start with 1 and work my way down the list until the wideband is close to the right reading.

All steps do different things, but also affect the overall function of the carb. So, every change you will need to go back and set idle speed and then begin your testing.